1. Field of the Invention
This invention relates to a low-distortion radio-frequency (including microwave and millimeterwave frequencies) amplifying apparatus equipped with a distortion compensating circuit for compensating a distortion generated due to the non-linearity of input and output characteristics of a radio-frequency amplifier.
2. Description of the Related Art
FIG. 7 of the accompanying drawings shows a conventional low-distortion radio-frequency amplifying circuit disclosed in, for example, Japanese Patent Laid-Open Publication No. 5240/1977. In FIG. 7, reference numeral 1 designates an input terminal; 2, an output terminal; 10, a distortion compensating circuit; and 40, a solid state power amplifier. The distortion compensating circuit 10 includes a divider 12 for distributing an input signal to each of three receiving elements, a distortion generating amplifier 20, linear amplifiers 21, 22, variable attenuators 25a, 25b, 25c, 25d, a distortion extracting combiner 30 for combining the output of the distortion generating amplifier 20 and the output of the linear amplifier 21 in opposite phases to extract a distortion, and a combiner 31 for combining the distortion component extracted by the distortion extracting combiner 30 and the output of the linear amplifier 22 in opposite phases.
In operation, an input signal inputted to the input terminal 1 is divided into the three amplifiers 20, 21, 22 by the divider 12. A first output of the divider 12 is introduced to the distortion generating amplifier 20. The signal, with a distortion component, amplified by the distortion generating amplifier 20 is introduced to the distortion extracting combiner 30.
A second output of the divider 12 is introduced to the linear amplifier 21 via the variable attenuator 25b and is thereby amplified without generating a distortion component, whereupon this amplified output is introduced to the distortion extracting combiner 30. In the distortion extracting combiner 30, the output of the distortion generating amplifier 20, which output contains a distortion component, is combined with the output of the linear amplifier 21, which output contains no distortion component, in opposite phases. Thus only the distortion component of the output of the distortion generating amplifier 20 is extracted, and this extracted output is introduced to the combiner 31 via the variable attenuator 25c. When extracting a distortion, the amount of attenuation of the variable attenuator 25a is set in such a manner that the amplitudes of the two signal components to be combined would be equal.
A third output of the divider 12 is introduced to the linear amplifier 22 via the variable attenuator 25d and is thereby amplified without containing a distortion component, whereupon this amplified output is introduced to the combiner 31.
In the combiner 31, the output of the distortion extracting combiner 30, which output is attenuated by the variable attenuator 25c, is combined with the output of the linear amplifier 22 in opposite phases.
The output of the distortion compensating circuit 10 is inputted to the solid state power amplifier 40 where amplification is performed so as to cancel a distortion component generated in the solid state power amplifier 40. This resulting output is outputted to the output terminal 2.
Therefore, for compensating a distortion, the amount of distortion generated in the distortion generating amplifier 20 of the distortion compensating circuit 10 must normally be equal to the amount of distortion generated in the solid state power amplifier 40, irrespective of the output power of the solid state power amplifier 40.
FIG. 8 of the accompanying drawings is a graph showing the output power dependency of a 3rd intermodulation distortion contained in the output of the solid state power amplifier 40. In this graph, a solid line represents a characteristic in the presence of the solid state power amplifier 40 alone, and a dotted line represents a characteristic when the solid state power amplifier 40 is equipped with the distortion compensating circuit 10. As is apparent from FIG. 8, the distortion near a particular output power can be compensated, but it is difficult to compensate a distortion over a wide dynamic range. This is because the amount of distortion generated in the distortion generating amplifier 20 cannot be equal to the amount of distortion generated in the solid state power amplifier 40 over a wide dynamic range.
With this conventional low-distortion radio-frequency amplifying ciruit, it is difficult to compensate a distortion generated in the solid state power amplifier over a wide dynamic range.